Microfluidic processes often employ the use of an emulsion, which contains drops of a dispersed liquid phase surrounded by an immiscible continuous liquid phase. Drops may be used as reaction vessels for chemical or biological reactions, as storage vessels, and/or as a method to isolate and compartmentalize molecules, such as chemical or biological elements. With proper chemistry such as surfactants on the surface of the emulsion, drops may be made “stable,” meaning they are substantially prevented form mixing and merging when in contact with each other. This stability allows one to create a population or library of drops composed of different chemical or biological components that may be stored in the approximately same volume of space without mixing or contamination between and/or among the components of one drop and another.
Currently, drops flow within a microfluidic device where many drops are injected with sample from a single large drop (herein referred to interchangeably as either a “large drop” or “slug”) or one more discrete inline samples. Often times “sets” of drops are created, meaning that a sequential number of drops will be injected with the same type of larger drop, resulting in a set of substantially similar drops. It is possible to have many sets of drops within a given sample. As the newly injected drops flow throughout the microfluidic channels within the microfluidic device, the drops often mix and rearrange themselves, resulting in the loss of defined sets of drops, causing issues downstream in detection and data analysis.
In some cases, it can be advantageous to label emulsion drops in a microfluidic network with a sample of interest in an alternating fashion. Doing so ensures that they are distinguishable when the drops proceed to a subsequent detection process, without detection overlap between adjacent drops. For example, sets of drops as discussed above can be distinguished by having different labels or by one set having a label and the other set lacking the label.
Microfluidic devices can use liquid emulsions comprised of a continuous phase and a dispersed phase, wherein the dispersed phase may include droplets that serve as vessels within which chemical or biological reactions may be performed. To perform these reactions, different functions can be performed on the droplets, such as the loading of reagents and reaction components (e.g., cells, proteins and nucleic acids) into the droplets, followed by incubating, sorting and/or optically detecting the droplets. For example, when introducing multiple reagents into one or more droplets, the volume of the continuous phase between droplets can be such that reagents are injected into any given droplet sequentially, i.e., one at a time rather than more than one at a time to avoid cross-contamination. Conversely, when storing the droplets in an incubation chamber or passing the droplets through an incubation channel, the volume fraction of the continuous phase must be reduced such that the droplets are packed closely and move in unison at a constant velocity, as well as to minimize the amount of space required within the microfluidic device.
These and other issues are addressed herein.